Hydraulic linear actuators are known which are used in hydraulic vehicle braking systems, in particular also in brake-by-wire braking systems. Such a hydraulic linear actuator is known for example from DE 10 2009 019 209, which is incorporated by reference. This linear actuator consists of a rotation-translation gear formed as a ball-screw drive and having a threaded spindle and an associated threaded nut, and of a brushless electric motor with a rotor and a stator which are arranged in a housing. The threaded spindle constitutes the drive shaft of the electric motor and is therefore driven directly thereby, while the threaded nut is arranged non-rotatably on a hydraulic piston-cylinder arrangement which forms the master cylinder of a vehicle braking system.
Such a hydraulic linear actuator may however fail on occurrence of a single fault, such as for example a blockade of the ball-screw drive. Operation of the braking system is then no longer possible so that the driver must brake in fall-back level. On vehicles with autonomous driver functions such as e.g. brake-by-wire systems however, the driver does not have or only has restricted access to a fall-back level.
To solve such a problem, DE 11 2005 003 675 T5, which is incorporated by reference, proposes a fault-tolerant linear actuator which is constructed redundantly with regard to components important for function. This known linear actuator thus comprises a first motorized drive device with a first motor and assigned thereto a first rotatable ball nut provided with ball bearings, a second motorized drive device with a second motor and an assigned second rotatable ball nut provided with ball bearings, and a single ball spindle which is in engagement with both the first and the second ball nuts. The ball spindle moves when one of the motors drives the ball nut or when both motors drive the ball nuts. The two drive devices are each arranged at one end of the single ball spindle so that the drive shafts of the two motors are oriented parallel and offset to the ball spindle. The ball nuts are each driven by the associated motor via a gear.
The disadvantage with this known fault-tolerant linear actuator can be regarded as its design which, because of the axially parallel arrangement of the two drive units and the ball spindle, requires substantial installation space and is therefore unsuitable for use in vehicle construction.
DE 20 2010 016 542 U1, which is incorporated by reference, discloses a solution for a compact spindle drive for a machine tool which comprises a first drive with a first motor and a second drive with a second motor, wherein a spindle shaft can be driven optionally by means of the first drive and/or the second drive, and the motor axes of the drives run coaxial to the spindle shaft. The compact structure results because the second drive at least partially surrounds the first drive. This is achieved in that the first drive is configured as a direct drive with a rotor coupled rotationally fixedly to the spindle shaft or as an internal rotor. The second drive is also configured as a direct drive in the form of an external rotor motor with a rotor which can be coupled to the spindle shaft via a clutch.